Electron discharge device



APlr'l 4, 1950 J. A. MoRToN ET AL 2,502,531

ELECTRON DISCHARGE DEVICE Filed Jan. 6, 1949 2 Sheets-Sheet l 4 J. A. MORTO/v NVENTg/S RMRVDER ORNEV April 4, 1950 v J. A MORTON ETAL 2,502,531

ELECTRON DISCHARGE DEVICE Filed Jan. 6, 1949 2 Sheets-Sheet 2 F IG. 4

x/a (xlcoNsTANr) J. A. MORTON A TTOR/VEV Patented Apr. 4, 1950 ELECTRON DISCHARGE DEVICE Jack A. Morton, Neshanic Station, and Robert M.

Ryder, SummitN. J., assignors to Bell Telephone Laboratories, Incorporated, New York,

N. Y., a corporation of New York Application January 6, 1949, Serial No. 69,564

4 claims. (ci. 315-5) This invention relatesfto electron discharge A devices of the space charge type and morev particularly to such devices especially suitable for fuse as broad band microwave amplifiers.

Two commony performance desiderata for vamplifiers are large gain and substantial uniformity of gain over avwide range of frequencies. -As will be pointed out presently, a figure of'merit 'fori-evaluating the performance of an4 amplifier is the gain-band product, which may be defined broadly here as a measure of the frequency bandl over which a gain of at least a specified magnitude is obtained. f

For relatively low frequencies, including frequencies corresponding to wavelengths of a substantial part of a meter, the device parameters and-fthe correlation thereof determining the gain `and vthe gain variation with frequency are now fairly well known. However, for frequencies in the microwave region, say frequencies of about 1000 megacycles and higher, although many ofthe device parameters and the broad-qualitative relation thereof entering into the determination of the performance maybe known, the operating characteristics of microwave space charge Vtype discharge devices of presently known construcvtions are far below obviously desirable standards.

lForl example, to the best of applicants knowledge,

the maximum gain-band product for the best devices of prior art constructionsk is about 65 megacycles per second at frequencies of the order of 1000 megacycles, theY gain being of the order of 13 decibels at 1000 megacycles and 3 decibels down at 998.4 and 1001.6 megacycles. One general object of this invention is to improve the performance of space charge type electron discharge devices. More specifically, one

'object of this invention is to attain a substan- Vtially maximum gain-band product for broad band microwave amplifiers.

In one illustrative embodiment of this invention, a discharge device comprises a cathode, a

control electrode or grid and an anode having juxtaposed parallel plane surfaces.` The device vis particularly adapted for use in the so called lgrounded grid or grid return type of circuit, with lwave-guide input and output circuits.

It has been ascertained that the performance of such a device is determined uniquely by the transadmittance, active and passive output capacitance, input conductance, and input loss conductance. It `has been ascertained further that the physical parameters of the discharge device can be correlated consistent with practical considerations and mechanical and thermal limita- 2 tions Vso that a maximum ligure of merit, specifically a maximum gain-band product is realized.

The improved performance obtained by the invention may be appreciated from the fact that inl specific devices of the construction hereinafter' described gain-band products of about 1800 megacycles per second at frequencies of the order of 4000 megacycles per second, the gain being at 4000 megacycles and 3 decibels down at 3910 and 4090 megacycles, have been obtained consistently. i f l In accordance with one broad feature of this invention, in a space charge type wide band amplier, the significant parameters are correlated so that thel gain-band product is substantially maximized whereby optimum performance is realized.

In accordance with another feature of this in'- vention, insuch a discharge device, the cathode current density and the electrode construction and spacing are made such that the electronic figure of merit, as hereinafter defined, is substantiallyamaximum.

In accordance with a further feature of the invention, is a discharge device of the general vconstruction described hereinabove, the portion of the enclosing vessel significantly affecting the passive part of the output capacitance is so con- -structedand associated with the anode that the ratio of the passive to active parts of the output capacitance is minimized. f

The'invention and the above-noted and other features of this invention will be understood more clearly and fully from the following detailed description with reference to the accompanying drawing in which:

Fig. l is an elevational view, partly in section of an electron discharge device and associated circuit coupling elements therefor, illustrative of one embodiment of this invention;

Fig. 2 is a plan view of the apparatusillustrated Fig. 3 isa fragmentary sectional view to an enlarged scale showing details of a portion of the discharge device and parts of the coupling to the input and output of the device;

Fig. 4 is a diagram illustrating one manner of association of the device and input and output wave guides therefor. 5 z.

Fig.r5 is a block diagram analog of thecoinbination illustrated in Fig. 4;.

Fig. 6'is a schematic or equivalent circuit representation of this combination; and

Figs. 7 to l0 inclusive are graphs showing qualitatively the relation between certain parameters and a performance criterion for an electron discharge device of the construction illustrated in Fig. 1.

Referring now to the drawing, the electron discharge device illustrated in Fig. 1 is of the parallel plane electrode type adapted for grounded grid operation and lcomprises an evacuated enclosing vessel having a metallic portion I0, a cylindrical insulating portion II hermetically bonded or sealed to a. flange i2 on the metallic ,portion I 0 and a dished insulating portion I3, the several por# tions being in coaxial relation. The cylindrical and dished insulating portions I-I and I3 are bonded or sealed hermetically to opposite faces of an annular metallic disc I4, for example of Kovar, through which electrical yconnection .mayvr be established to the control electrode or grid, as will appear presently. The metallic portion ID is' joined hermetically to a base I5 having terminals I6 vtherein through which connection -to the cathode maybe made.

Mounted within the enclosing vessel is a unitary assembly, for example of the construction -disclosed in the application Serial No. 777,312, led October 1, 1947 of J. A. Morton and L. J. Speck, now Patent No. 2,455,381 granted Dec. 7, 1948 which comprises a pair of ceramic members I'I and I 8 and a cathode supported from the member Il. As shown clearly in Fig. 3, the cathode includes a stem, for example of molybdenum :having a disc head portion I9 to which a nickel disc 2U is .aixed The face of the disc 2li is coplanar with the surface 2I lof the insulating member III and is coated with a good electron emitting material, such as a mixture of barium and `strontium oxides. The cathode and the heater element therefor, not shown, are connected to the terminal prongs I6 by suitable conductors, 4also :not

shown. A radio frequency "connection is Iestablished between the cathode and the vessel portion lII'I nby an annular metallic member Y22 -from which a metallic cylinder 23, coaxial with and in juxtaposition to the Vessel portion I0 and dening -a Icondenser therewith, depends.

Seated upon an annular spacer 24, .in turn v.seated upon the `surface ZI is a control electrode Aor grid which comprises an annular disc and .parallel wires 26 extending across the aperture in the disc and-equally spaced from the emissive Y `face of the cathode. The grid frame or -disc 25 bears against a metallic ring :27 which Vvis .joined tothe disc I4, whereby electr-ical connection tothe .grid or control electrode maybe established.

The anode comprises a stem 2-3 hermetically l'sealed to the insulating portion 13 of the -enclosing vessel, and a button 29 having its face ktoward the grid and cathode planar and parallel thereto. The stem 28 may be of Kovarand the button of copper.

The electron discharge device `is positioned `inra `cylindrical recess in the body 33 of a 'metallic coupler, the vessel I0 being held coaxia'lly within `the recess by an 'annular metallic spring member V4I) fitted to the Vessel and the wal-lfofl the recess :surrounding the vessel and the disc I=4 bearing 35. 'Suitable waveguides, not fs-hown iin Fig. 1,

may be coupled to the inputr-and-'foutput'windows '132 'and 33 respectively.

The input coupling including the window 32 and cavity 34 may be tuned and the coupling adjusted by a single screw 38 threaded into the body 30, projecting into the window 32 and lon- 5 gitudinally movable against the action of a lever spring 3l xed adjacent one end by a screw 38.

The output coupling including the window 33.

and cavity similarly may be tuned and the coupling adjusted by a single screw 39 extending into the window 33 and acting against a second lever spring 31.

Associated with the anode is a tuner and choke -assembly including coaxial cylindrical metallic members 4I', 42, 43 and 44, for tuning the gridanode cavity. ASpecically, the anode stem 28 is closely Aiitted into a central aperture in the end of the member 4I, the latter being supported from an insulating disc 46 which is fixed upon the `outer end of the member 44. The member 43 carries the choke member 42 and is ntted slidably -withint'he outer member 44. 1t is movable 'longitudinally by rotation of the adjusting nut .4] threaded to the outer member 44 .and coupled to the member 43 by screws 48 which are-slidable in slots 49 in the outer member. The rjuxtaposed portions of the members 4I, 42 and 43 define .a quarter wavelength choke `joint which serves to prevent lossv of energy from the output cavity. The latter is tunable by `motion of the members #S3 and 45.

An amplifier including a ldevice `such as .illustrated in Fig. land appropriate inputand output circuits may be considered as composed .of three transducers in tandem yas illustrated .in Figs. 4

and 5. The input transducer T1 extends from a region )3--0 in the yinput wave guide 53 A-iivhere `only the dominant wave exists tozimmediately ad- .jacent theelectron stream-at the region ,I--I at the grid-cathode gap. It .includes-the external input Icavity portion and that portion of .the vdischarge device up to the region .II.. .Similar-ly, the'output transducer T3 extends from a Aregion VIi---Sf -in the output wave .guide 5I where'cnly the dominant Wave exists Vto .immediately .adjacent the electron stream at the region 2-2 4:at

the grid-anode gap. tIt includes the external .output resonator -portion4 and that yportion of the dis- .charge device up to the region 2 2'.. .Theactive transducer T2 .includes the electron .str-earn Aand vthe active portions vof the electrodes, that is, those portions which interact `ydirectly `with Athe yelectron stream. t-extends fromtheregion l--I to fthe region 2-2..

For many practical cases, the combination of transducers may be consideredas inLEig. .6 wherein .the input and voutput 4transducers are .represented by ideal transformers `the turnsratios .of ywhich vare real'and independent .of irequencyand -the shunt vadmittance of the device -consists :of lumped constant circuitelements. This representation is permissible-for cases where.the .combined circuit has no series and l'only lone shunt lresonance near the operating .frequency .band. AIt isV notstrictly accurate :for cases such .as v:double tuned Vlcircuits having itwo l.resonances near the frequency band of interest. However, such complication of `the circuit may-be considered as .a circuit broad-bandinggproblem, .the .broad -banding producing an improvement which does not `depend upon 'the discharge-devicepernseif .there is no feedback. Thus, VAthe .sing-1e y.circuit ysonfguration 'may beused asa 'basis -for comparing performance 'of the discharge .devices even fin the complicated circuit cases.

-Two yInerformance :criteria for :ampliers are the gain and the band width, the latter being defined as the magnitude of the frequency interval Within which the vgain is within some specified range. The `band width as treated hereinafter will be considered as that interval within which the voltage gain is within a factor of times the maximum, the maximum occurring at the approximate midband frequency. In general, the form of the band characteristic is dependent upon the Q of the input and output circuits. If the Qs of these circuits are equal, that is, the band is shaped equally by the input and output circuits, the gain decreases 6 decibels per octave in band width; if the Q of the input circuit is much smaller than that of the output circuit, the gain decreases 3 decibels per octave 'of band widening.

In an amplifier including a discharge device such as illustrated in Fig. 1 and operated in a grounded grid or grid return type of circuit, the Q of the input circuit is ordinarily much smaller than that of the output circuit. For this case, it can be shown that the amplifier performance can be evaluated from the relation I |Y21l2 C22 Gn 2B-@Tnet caw...) GT+ G.. F (l) where 1 10=insertion voltage gain at midband frequency.

B='band width, 3 decibels down.

Y21=transfer admittance of the discharge device.

G11=input conductance of the discharge device.

Cz2=grid to anode capacitance of the discharge device.

Cp2'=passive part of output circuit capacitance.

Gp1=loss conductance in input circuit.

For capacitances in micromicrofarads and transadmittance in micromhos, the gain band product Io2B, is in megacycles per second.

It will be noted that in the right-hand side of the equation, the rst factor is the electronic gure of merit of the discharge device alone and is dependent upon only the parameters of the device; the second term is a degradation factor, due to the effect of adding passive capacitance to the active capacitance in the device; and the third term also is a degradation factor due to losses in the input circuit. The third factor can be minimized by the use of low loss insulating materials in the input path and low Iloss connections to the cathode and the grid.

The factor F is a band definition matching factor which depends only upon the characteristics of the input and output circuits and the band width. If the input and output circuits are matched and the band width is taken between the 3 decibels down points, this factor is unity.

The transfer admittance Y21 is substantially constant over band width usually employed; it varies slowly with frequency and approaches the transconductance at f 0. In a device of the construction illustrated and described, a transconductance of the order of 50,000 mmhos at an anode circuit of about 25 megacycles is obtained.

The product IU2B is a figure of merit for the amplifier, that is, the greater this product the better the performance or rating of the amplifier. As has been pointed out herinabove, the third term or factor on the right-hand side of Equation 1 can be maximized. A large value for the second term obtains if Cpu is small in comparison td C22. This will be discussed in some detail presently. The first term, as has also been pointed out hereinabove, depends upon only the parameters of the electron discharge region, all of which can be operated upon. Maximizing of this first term will be understood from the following consderations. Subject to the assumptions in and on the basis of the analysis for triodes presented in Vacuum Tu'be Networks by Llewellynvand Peterson, IRE Proceedings, March 1944, page 144, it can be shown that the intrinsic gainband product for the discharge device may be expressed in two forms, to Wit:

K and K' are functions only of frequency X1 is the cathode to grid spacing in centimeters 61 is the cathode to grid transit angle in radians and equals Laim i is the cathode current density in amp/cm.2 02 is the grid-anode transit angle in radians and equals 6300X2 x V, X2 is the grid to anode spacing in centimeters A is the wavelength in centimeters Vp is the anode direct voltage, and F1(01) F2092) and F3(01) are functions of the respective transit angles 01 and 02. y It will be noted that in Equation 2, the current density y' is involved only in the second factor on the right-hand side, this factor being a function of 01, which as indicated above, varies as The relationship between gain-band product and the current density y' is illustrated qualitatively in Fig. 7. As is evident from this gure, maxima for the gain-band product appear for several values of 01, the rst occurring at a point where 01 approaches 0, that is, where y is very large.`

The other maxima, it can be shown are of primary interest only in narrow frequency band cases and, hence, need not be investigated further here. Thus, the current'density should be as large as possible, consistent of course, with long cathode life. In a device of the construction illustrated in Fig. 1, a current density y of met/cm.2 has been found satisfactory, the cathode life being several thousand hours.

It will be noted further from Equation 3 that the cathode to grid spacing X1 is involved only in the second factor, which is a function of 01. The relationship between gain-band product `and X1 is illustrated qualitatively in Fig. 8 from which it will be seen that, as in the case of Fig. "I, several maxima `are indicated. The first occurs where 01 approaches zero, which, of course, requires X1 to be very small. rIhe other maxima may be neglected here for the same reasons noted in the discussion of Fig. 7.

As a practical matter, it is mechanically impossible to make the parameter X1 zero. Thus, in anactualdevice the cathodeto grid spacing and the ratio where P is the pitch.-distance-betweenA the grid'wire centers, and d is the grid wire diameter;

The relation between gain-band product and this function is illustratedv qualitatively in Fig. 1,0, wherein the plots are for a fixed value of X1. From this, figure, .it will. be. noted that.. the grid dimensions P and dshould. heY as.v small as. mechanically possible and in relation to produce a large Value of a, specifically no less than 0.5.

In a typical device of, the construction illustrated in Fig. 1, the optimum relations consistent with: practical; limitations. are: realized'V with; a spacing X1 of 0;000T inch', aigridpitclrl? of 0.061 inch, and a grid: wirel diameter of' 0.00031 inch. For these values, it willbe not'ed,.thegrid .transmission actor is 0.7.

From relations 2- andv 3. itv will be noted that the anode to grid spacing X2 is involved only in the.` third factor, which. is a. function: of the output transit angle 02. The relation between gainband product and X2 is illustrated qualitatively in Fig. 9. It is characterized by a deiinite maximum which occurs at 02:2;9 radians. This relatively lar-ge-fvalue of 62 for the maximum gain.-l band productisexplainable by. the factsV that the capacitance C22 varies as in Equation 1 increases until the spacing X2 becomes relatively large, that is, as H2 approaches 2.9 radians; It will he understood', ofcourse, that 62 depends upon the anode voltage and'theoperating frequency. For example, in a device-of the construction illustrated' in Fig. 1 for a voltage of 250 volts and a frequency of. 400`0fmegacycles per second, the optimum yspacing X2 is 0.025211inch. A large-spacing may beadvantageous in-that-by virtue of the correspondingly small capacitance, a high maximum frequency limit on the device is attainable; However, the circuit degradation factor, that is, the second factor on the righthand. side of Equation 1, maybecome. enough larger to aiect the smaller Cnasthev capacitance Ca2 isvdecreased hy increasing the. spacing Xt.

Si Also',- if: the spacing is increased andi the'- anode' Voltage maintained constant; in'A order that the anode may draw the; desired current, it maybe necessary thatl the grid be operatedl positive so thatiexcessve grid' current would obtain. Thus, in the construction! oan actual device a number oi' practical considerations must be balanced in correlating the factors entering into the determination of the optimum spacing X2. Some departure from the optimum'va-l'ueA of 02 Without too great a decrease in the gain-bandi product maybe tolerated in some cases. As an indication of" the decrease', it'maybe/notedv that in a' device of the construction illustratedi'n Fig. l4 for which the optimumwalues" of 92 and' X2 were asv given above, Values ofc 02 and X2 of'V 1.6 radians and 0.012` inch respectively resulted in 78 per. cent ofthe theoretical". maximum gain-band product;

From the' foregoing it is evident that theY rst factor in the right-hand side ofi Equation l 'can be maximized' by correlation' of' theY parameters of the discharge. device. general, tosurnmarize the` cathode current'r density should be as large as possible, the grid-cathode spacing should be as small asis mechanically practical, the grid transmission factor, should be.large, and the'out.- put transit angle should be of the order of 2.9 radians.

lt willbeI appreciated that maximizing of the rst term on the right-hand side of Equation 1 consistent with the considerations mentioned, determines the value of the parameter. C'22, which occurs in the second. term as well, as in the. first. It is apparent'v thata Alarge valuefor this second term requires that Cp; be smallv in comparison With C22.

The term Cpz, as has been indicated hereinabove, is the passive part of the output circuit capacitance. A principal element entering into the determinationvoifvr the-magnitude off this term is the insulating or dielectric portion Iv3-of the enclosing'vessel: Thisaportion, it will-be noted, is inthe output resonator 352 The energy ywhich willl be stored in; the-dielectric por-tionr I3 is a function of-'the coupling between this portionand the electricfield' within the-resonator 35 To obtain a low value for thisV energyY and, hence', a small value forl Cpe, the insulating member |-3 Where it extends substantially parallelI to the electric linesoftheleld within the resonator'should he at a region of low eldintensity.y This'condition is attained by-the conguration oft the memlo'er I3" illustratedv inv Figs; land 3 together with the sealingof the member'` to the.` anode stem 28 J adjacent the point upon'thestem at which a voltage node appears during operation of"l the device. The eldlin'es are essentially normal tothe juxtaposed faces of', the anode` and grid at the output gap. However, beyond the'gap theV eld'lines are curved, speci'call'y concave downward' with re.- spect. to. the gap,` and. the iorrnv thereof in. the region. of. the. insulating. member I3. closelyl ap.- proximates the, conguration of themember` I3 shownin the-drawing.

What is` claimed is.:

1. An.L electron discharge device` comprising a control electrode structure, including.- a `planar grid, a cathode and an anode on opposite sides of. said grid and having plane.facesinjuxtaposition-thereto, the cathode current density andthe electrode spacingsfbeing such-.thatfor the device the ratio:

is substantially a maximum, Ym being the transfer admittance, G11 the input conductance and C22 the grid to anode capacitance, means including said anode and grid defining an output resonator chamber, and a vessel enclosing said grid, cathode and anode including an insulating portion in said chamber-1 extending between said anode and said control electrode structure, the configuration of said linsulating portion being such that the ratio is substantially a maximum, CP2 being the passive 'part of the output capacitance.

2. A broad band microwave amplier of the grounded grid type comprising a cathode having a plane electron emissive face capable when heated of producing an electron stream of density of substantially 180 maycm, a control electrode having a plane grid portion opposite and parallel y to said face and spaced of the order of 0.0007 inch therefrom, said gridv portion having a transmission factor of the orderu of 0.7, and an anode having a plane surface, opposite and parallel to said grid portion and spaced therefrom a distance 10 I 4. A broad band microwaveamplifier of th grounded grid type comprising an enclosing vesselfi'ricluding a dished insulating portion and a slubs ntially cylindrical insulating portion havl, e end opposite the edge of said dished portion;V n annular metallic member between said ins'lyating portions, said one end and said edge being sealed to opposite faces of said metallic member, a grid extending over the aperture in said-metallic member and electrically connected vt s'aid member, said grid having a transmission factor of the order of 0.7, a cathode having a 'fplaltie emissive face opposite one face of said grid -andjspaced therefrom of thev order of 0.0007 inch, 15 :means including said cathode and grid defining fan input chamber resonator, an anode including a stem extending through the base of said dished 4portion and sealed thereto, said anode having a v.plane face opposite the other face of said grid and spaced therefrom a distance such that in the operating band of the amplifier the grid to anode transit angle is of the order of 2.9 radians, and means including said anode and grid defining an output chamber resonator, said dished member conforming substantially to the electric fields in said output resonator at the region of said member, and the seal between said stem and said dished member being at substantially a voltage no'de region on said stem.

' JACK A. MORTON.

ROBERT M. `RYDER.

REFERENCES CITED The following references are ofrecord in the file of this patent:

UNITED STATES PATENTS Number Name Date .2,424,089 l,Cvrethma'nn July 15, 1947 2,428,609 Beggsv Oct. 7, 1947 2,465,801 Gurewitsch Mar. 29, 1949 

